The invention relates to a method for correcting the measuring result of a coordinate measuring apparatus wherein a large piece is continuously scanned and a coordinate measuring apparatus for carrying out the method.
A method of this kind is disclosed in U.S. Pat. No. 5,594,668. Here, the dynamic stiffness of a probe is to be determined in that first a teaching ring or a calibration ball is scanned at different velocities with a very stiff probe. Thereafter, the teaching ring or the calibration ball is scanned with the probe, which is to be tested, in the same manner as with the very stiff probe. The dynamic stiffness is determined from the differences of the measurement values of the very stiff probe and of the probe to be calibrated. From this, and in a later measuring sequence for the probe, corresponding corrective values are determined while considering the acceleration of the probe and the measurement results are correspondingly corrected.
The described method has provided good results in the past. However, it has been shown that measurement errors, which result from the dynamic bending of probes can be corrected only to a limited extent with the described method for the continuously increasing demands as to accuracy.
It is an object of the invention to provide a method for correcting measurement errors which result because of the dynamic deformation of the probe pin when scanning a workpiece. It is another object of the invention to provide a coordinate measuring apparatus for carrying out the method of the invention.
The method of the invention is for correcting a measurement result of a coordinate measuring apparatus wherein a workpiece is continuously scanned with a probe having a dynamic bending characteristic. The method includes the steps of: determining a parameter field defining the dynamic bending characteristic of the probe from at least one of the following: a product of the static bending tensor (NT) of the probe and the mass tensor (MT+mE) of the probe; and, deviations accompanying an acceleration of the probe normal to the surface of the workpiece; computing corrective values from the parameter field while considering the acceleration ({right arrow over (b)}) of the probe; and, correcting the measurement with the corrective values.
According to a feature of the method of the invention, the parameter field, which defines the dynamic stiffness of the probe, is described by the product of the static bending tensor of the probe and the mass tensor of the probe and/or, specifically, the parameter field is defined by the deviations with the acceleration of the probe normal to the workpiece surface.
Considerably better measuring results are obtained by the correction of the measuring results with a corresponding parameter field. A very good measuring result results when the parameter field defines both of the above-mentioned characteristics, that is, that the parameter field includes a component field which is the product of the static bending tensor of the probe and the mass tensor of the probe as well as a component field which defines only the deviations with the acceleration of the probe normal to the workpiece surface.
The measuring results can additionally be improved slightly when the parameter field additionally also includes a component field which describes only the deviations for tangential acceleration of the probe relative to the workpiece surface.
The parameter field, that is, the parameters of the parameter field, can here be determined either by an analytic computation and/or by dynamic calibration. It is here to be noted that, for the parameter field, only at least one of the component fields need be calibrated for correction, especially when the parameter field consists of several component fields.
For the case that the parameter field defines the deviations for the acceleration of the probe normal to the workpiece surface, the parameters of the parameter field can be advantageously determined by continuous scanning of a rotationally-symmetrical calibration body having different velocities. The rotationally-symmetrical calibration body can advantageously be a calibration ball. At least three large circles are scanned at different speeds for determining the parameters of the parameter field.
When the parameter field additionally defines the deviations for tangential acceleration of the probe relative to the workpiece surface, the parameters of the parameter field can be determined in that a curved path is scanned on a calibration plane which is aligned parallel to the workpiece surfaces to be measured. The scanning takes place in accordance with at least one of the following principles:
a) one and the same curved path is scanned at different velocities; or,
b) the path is scanned at a fixed velocity and the path has different curvatures.
The parameters of the parameter field can be determined by measuring a small circle on a calibration ball at different speeds in at least one calibration plane.
The measuring points, which are recorded during calibration or are recorded in later measuring operations, can be validated as valid or invalid in dependence upon the measured acceleration and/or the measured measurement force. For validation, an angle between the acceleration vector or measurement force vector and the normal vector of the workpiece surface can be computed at the measurement point and a measurement point can be validated as being valid when the angle exceeds or drops below a previously defined value.
The maximum permissible measuring velocity of the coordinate measuring apparatus can be advantageously determined while considering the dynamic stiffness of the probe and of the probe mass.